The present invention relates to an automotive air conditioning system and, more particularly, to an automotive air conditioning system with automatic control of refrigeration compressor capacity and inlet air mixing door.
Automotive air conditioning systems that include a duct for introducing air into a cabin and a refrigeration circuit comprising a compressor and an evaporator are known in the art. In a typical automotive air conditioning system, the speed of the compressor is mechanically linked to the speed of an internal combustion engine. A problem encountered in these systems is that continuous operation of the compressor, especially during low cooling demand, causes the temperature of the evaporator to become extremely cold, allowing moisture in contact with the core to turn into ice. This xe2x80x9cfrosting upxe2x80x9d inhibits air flow through the evaporator causing the cooling capacity of the air conditioning system to deteriorate, which compromises passenger comfort.
To prevent the evaporator from becoming too cold, typical automotive air conditioning systems include a means to control the capacity of the compressor. In conventional automotive air conditioning systems, the capacity of the compressor is generally controlled by (i) varying the displacement of a variable displacement compressor or (ii) modulating the duty cycle of a clutch driven compressor. Determining whether to modify the capacity of the compressor requires an evaluation of the evaporator operating conditions.
One evaluation approach is to attach a temperature sensitive device, such as a thermistor, directly to the evaporator to measure the temperature of the evaporator. A control unit is provided for switching off or varying the displacement of a compressor when the detected temperature of the evaporator is lower than a predetermined value to prevent the evaporator from xe2x80x9cfrosting-up.xe2x80x9d A limitation of this approach is that the thermistor measures the temperature at a finite point on the surface of the evaporator, permitting the control system to turn off or vary the displacement of the compressor when only small portion of the evaporator is too cold.
Another evaluation approach is to insert a pressure sensitive device, such as a pressure switch, into a refrigeration line entering or exiting the evaporator to measure the pressure of the evaporator. As the evaporator cools, the pressure of the refrigerant circulating through the evaporator decreases. The control unit switches off or varies the displacement of the compressor when the detected pressure of the evaporator is lower than a predetermined value to prevent the evaporator from xe2x80x9cfrosting-up.xe2x80x9d A limitation of this approach is that the pressure switch measures the pressure at a finite point in the refrigeration circuit. As the surface of the evaporator becomes contaminated with debris in the duct, the relationship between the pressure (temperature) of the refrigerant and the surface temperature of the evaporator changes allowing the control system to turn off or vary the displacement of the compressor prematurely.
Another problem encountered in a typical automotive air conditioning system is when the heat load on the vehicle exceeds the cooling capacity of the air conditioning system. Once the cooling capacity has been exceeded, passenger comfort can no longer be guaranteed. Factors that contribute to insufficient cooling capacity can be characterized in two basic categories: environmental factors and mechanical factors. The primary environmental factors are the outside air temperature, humidity of the outside air and solar load on the vehicle. The primary mechanical factors are compressor speed, effectiveness of the evaporator and air flow through the evaporator. Numerous combinations of these factors help contribute to the heat load on a vehicle exceeding the cooling capacity of the vehicle air conditioning system.
The cooling capacity of an air conditioning system can be improved if a portion of the unconditioned (outside) air flow over the evaporator is replaced with conditioned (re-circulated) air from the vehicle cabin. Because conditioned air is generally cooler and dryer than unconditioned air, the use of conditioned air can reduce the load on the air conditioning system. However, excessive use of conditioned air causes the vehicle cabin to become too dry, leading to passenger discomfort, such as, for example, dry eyes. Therefore, many automotive air conditioning systems are designed to use only enough conditioned air as needed for the system to meet the heat load on the vehicle.
In typical air conditioning systems, an air mix door is used to regulate the amount of conditioned air that is mixed with the unconditioned (outside) air. The control unit used to control the capacity of the compressor may also function to regulate the position of the air mix door and, correspondingly, the amount of conditioned air flowing through the evaporator. The control unit typically employs a temperature/pressure sensitive device, as described above, to determine if the evaporator is too warm, indicating the heat load on the vehicle has exceeded the cooling capacity. As described above, a limitation of this approach is that the system is unable to ascertain the temperature of the entire evaporator because conventional temperature sensitive devices measure the temperature at a finite point on the surface of the evaporator and conventional pressure sensitive devices measure the pressure at a finite point in the refrigeration circuit.
An automotive air conditioning system is provided comprising a duct for introducing a flow of air into a vehicle cabin and a closed-loop refrigeration circuit including a compressor for circulating a refrigerant through the circuit and an evaporator arranged in the duct to contact the air flow such that an exchange of heat occurs between refrigerant in the evaporator and the air in the duct. The air conditioning system further includes a temperature sensor arranged in the duct for measuring the temperature of the evaporator and a control unit for selectively commanding operation of the system in accordance with the measured temperature of the evaporator. The duct includes an outside air inlet and an inside air inlet separated by an air mix door that regulates the amount of inside (re-circulated) air introduced into the duct.
In a preferred embodiment, the temperature sensor comprises an infrared (IR) temperature sensor that is preferably positioned downstream of the evaporator to remotely measure the temperature of a downstream surface of the evaporator. The use of an infrared (IR) temperature sensor allows the temperature of substantially the entire downstream surface of the evaporator to be measured instead of a small finite area of the evaporator. The sensor transmits an output signal to the control unit, which corresponds to an average measured temperature of the downstream surface of the evaporator. The control unit processes the output signal and initiates a control function to control operation of the air conditioning system in accordance with the measured temperature of the evaporator.
According to one control function, the control unit compares the measured temperature of the evaporator to a first predetermined reference temperature. If the measured evaporator temperature falls below the first reference temperature, the control unit reduces the cooling capacity of the refrigeration circuit to prevent a xe2x80x9cfrosting upxe2x80x9d of the evaporator. The control unit reduces the cooling capacity of the refrigeration circuit by reducing the capacity of the compressor, such as by shutting off or reducing the displacement of the compressor.
According to another control function, the control unit compares the measured temperature of the evaporator to a second predetermined reference temperature. If the measured evaporator temperature exceeds the second reference temperature, the control unit adjusts the position of the air mix door to increase the amount of conditioned air re-circulated through the evaporator, thereby reducing the heat load on the system.
The use of a an infrared (IR) sensor advantageously enables the control unit to perform a control function based on the temperature of substantially an entire surface of evaporator 16, as opposed to a small finite area of the evaporator. Among other advantages, the product development time of a vehicle employing the inventive air conditioning system is substantially reduced because extensive testing is no longer required to determine the best location of conventional sensors. Environmental factors, such as changes in elevation and atmospheric pressure, which impact the performance of conventional air conditioning system sensors, have no impact on an infrared sensor. Since an infrared sensor examines a substantial portion of the evaporator surface, the compressor is permitted to operate at maximum capacity until a significant portion of the evaporator is deemed too cold. Because an infrared sensor is a non-contacting measurement device, corrosion and contamination on the surface of the evaporator will generally not effect the temperature measurement.